Pressurized combustion and heat transfer process and apparatus

ABSTRACT

The present invention is concerned with combustion and heat transfer processes and apparatus. The invention has general applicability in the fields of combustion and heat transfer and is applicable to industrial and non-industrial processes as well as residential use. Practical industrial application of the invention may be found in the field of steam generation for heating and for electrical power generation. In addition, non-industrial applications of the invention include cooking appliances, stoves, water heaters, furnaces and the like.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority benefits of U.S. ProvisionalApplication Serial No. 60/138,009 filed Jun. 8, 1999 and the subsequentPCT filing of PCT/CA00/00658 filed Jun. 1, 2000 under the provisions of35 U.S.C. §119(e).

FIELD OF THE INVENTION

The present invention is concerned with combustion and heat transferprocesses and apparatus. The invention has general applicability in thefields of combustion and heat transfer and is applicable to industrialand non-industrial processes as well as residential use. Practicalindustrial application of the invention may be found in the field ofsteam generation for heating and for electrical power generation. Inaddition, non-industrial applications of the invention include cookingappliances, stoves, water heaters, furnaces and the like.

BACKGROUND OF THE INVENTION

Efficient use of heat generated from a fuel involves two fundamentalsteps. This first is the combustion of the fuel, and the second is theheat transfer from the products of combustion to the desired heat sink.Combustion processes are carried out so that the ambient temperature inthe combustion area is extremely high, i.e., typically greater than1500° C. It is well known that at high temperatures, nitrogen present infuel and air reacts with oxygen to forms various oxides, commonlyreferred to as NO_(x). The generation of NO_(x) increases with thetemperature, especially when an excess of oxygen is present. It istherefore desirable, when dealing with combustion of fuel, to maintaintemperatures as low as possible to inhibit the formation of pollutantslike NO_(x). An alternative is to reduce the concentration of oxygenbelow the stoichiometric requirement.

In many areas of the world, wood is still used as the main fuel forcooking. This is particularly true for so-called lesser-developedcountries where access to other fuels may not be readily available, oraffordable.

To inhibit the formation of pollutants during combustion, and toefficiently utilize available fuels, it is desirable to developappliances in which there is efficient combustion of the fuel andsimultaneous efficient heat transfer of the heat generated during thecombustion process to an appropriate heat sink.

SUMMARY OF THE INVENTION

In accordance with the invention, there is provided a heating apparatuscomprising a housing having a general axis. The apparatus furthercomprises a fuel support surface. The apparatus comprises a plurality ofair injectors arranged on the support surface. The air injectors have aplurality of apertures to deliver air in a first direction substantiallyparallel to the axis of the housing and in a second directionsubstantially normal to the axis of the housing. Fuel is burned adjacentthe fuel support surface. Air is injected with a fan from an air inletchamber to the air injectors. In addition, the heating apparatuspreferably comprises a restrictor ring placed within the housing abovethe fuel support surface to restrict the cross-sectional area of thehousing adjacent the restrictor ring. Further, the apparatus comprises asupport means for supporting a heat sink adjacent the upper portion ofthe combustion chamber. There is a thermal transfer gap between theupper edge of the combustion chamber and the lower edge of the heat sinkso that gases passing upwardly through the housing impinge upon the heatsink and pass through the thermal transfer gap after transferring theheat contained therein to the heat sink.

IN THE DRAWINGS

FIG. 1 is a vertical cross section of a heating device according to apreferred embodiment of the invention;

FIG. 2 is a plan view of the air injection system of the apparatus shownin FIG. 1;

FIG. 3 is a vertical, sectional view of an air injector;

FIG. 4 is an plan enlarged view similar to FIG. 2 showing the air flowpatterns from the injectors of FIG. 2;

FIG. 5 is a vertical, sectional view similar to FIG. 1 showing the airflow patterns within the apparatus of FIG. 1;

FIG. 6 is a cross-sectional view along the line A—A of FIG. 5;

FIG. 7 is a cross-sectional view along the line B—B of FIG. 5;

FIG. 8 is a vertical, sectional view of a further preferred embodimentaccording to the present invention.

FIG. 9 is a vertical, section view of another embodiment of the deviceof the present invention;

FIG. 10 is a second vertical sectional view of an air injector; and

FIG. 11 is a plan view of an air injection system of the deviceillustrated in FIG. 9.

DETAILED DESCRIPTION OF THE INVENTION

In order that the invention may be more clearly understood, reference ismade by way of example to a preferred embodiment of the invention whichis illustrated in the accompanying drawings.

The apparatus 10 illustrated in FIG. 1 is a heating apparatustransferring heat to a heat sink. The heating apparatus 10 is a woodfired cook stove transferring heat to a heat sink 12. In this case, theheat sink 12 is in the form of a cooking pan such as fry pan. Theparticular configuration of heat sink 12 to be heated by the apparatusdoes not form part of the invention, and can take any shape, whetherflat, concave or otherwise. However, the relationship between the heatsink and the relevant portions of the heating apparatus 10 are importantin the heat transfer process which will be discussed more fully below.

Heating apparatus 10 comprises an air inlet chamber 14, a housing 16 anda heat sink support 18. The air inlet chamber 14 comprises a fan 20driven by a conventional electric motor (not shown) that may be batterydriven, powered by an alternate electric source or by a windingmechanism supplying the required electric energy. Fan 20 draws airaxially into the lower portion of air inlet chamber 14 and directs theair to flow axially upwardly from air inlet chamber 14 into housing 16.Air inlet chamber 14 further comprises an adjustable air flow valve (notshown).

Housing 16 comprises a fuel support surface 30. Steps to reduce heatlosses through the housing 16 obviously increase the efficiency of theheating unit. Accordingly, it is preferred that the inner wall 36reflects radiated heat back to the combustion gases. A further tubularmember 120 is provided around housing 16, defining an area 122therebetween extending throughout the length of housing 16. Supportplate 30 is divided into 2 separate plates 124 and 125 inside housing 16between the combustion chamber and the air inlet chamber 14. Plate 124comprises a central opening 126 and a plurality of openings 128 aroundits circumference. Plate 125 is also provided with a central opening130. Plates 124 and 125 are coupled by a tubular member 129 that forms anarrow gap 127. Air is forced to pass through aperture 126 and impingeson plate 125, making an air stream 101 passing through gap 127 and mixeswith air coming through openings 128, thereby forming air stream 132before ultimately exiting through circular gap 134. Air also passesthrough aperture 130 and flows upwardly along with stream 100 (see FIGS.1 and 5.) Injectors 34 extend throughout both plates 124 and 125.

In a preferred embodiment, the fuel is a piece of wood that can beplaced on the support surface 30 which may be a flat generally circularplate (FIG. 2) or a generally flat square or rectangular plate (FIG.11), coupled to or sitting upon the air inlet chamber 14. Housing 16rests upon the fuel support surface 30. To assist in positioning thehousing 16 and tubular member 120, heat sink support 18 and the upperpart of tubular member 120 comprise each a slot for engagement therein,and an annular flat ring 37 sitting on structure supporting plate 39 isprovided to ensure proper positioning. Alternately, the fuel supportsurface 30 may include two or more bosses 32. The housing 16 ispositioned over the bosses 32 and lowered onto the fuel support surface30 where it is then aligned over the air inlet chamber 14. It should benoted that although wood is used as an example for fuel, other solidfuels like particulate fuels, powder fuels, liquid and gaseous fuels canalso be employed. More specific examples include coal, natural gas,gasoline, kerosene etc.

Referring to the embodiment illustrated in FIGS. 1, 5 and 9, the supportsurface 30 further comprises a plurality of air injectors 34 located ina substantially circular array. The diameter of the array is slightlysmaller than the interior diameter of housing 16 so that air injectors34 are located substantially adjacent to the inner surface 36 of housing16.

FIG. 3 illustrates a vertical cross-sectional view through an airinjector 34. The air injector comprises a body 40 comprising itself abore 42 extending longitudinally through the body 40 from an inlet end44 to a first outlet 46 and a second outlet 48. First and second outlets46 and 48 discharge air in directions that are substantiallyperpendicular to one another. First outlet 46 discharges air in adirection substantially parallel to axis 17 of housing 16. The secondoutlet 48 discharges air in a direction substantially perpendicular toaxis 17 of housing 16. Preferably, the diameter of second outlet 48 islarger than that of first outlet 46. First outlet 46 is considerablysmaller in cross-sectional area than bore 42 of injector 34, therebyensuring that the air discharged from the first outlet 46 is speeded upto exit at a relatively high velocity compared to the is velocity at theinlet 44.

Second outlet 48 is substantially adjacent to fuel support surface 30 sothat air discharged from that outlet travels across the fuel supportsurface 30 toward the fuel. In the embodiment of FIGS. 1 and 5, it willbe seen that injector 34 extends through plates 124 and 125 and thatsecond outlet 48 is substantially adjacent to plate 125 (see also FIG.10).

FIG. 4 illustrates the flow pattern from the second outlets 48 of airinjectors 34 arranged in a circular array. The air stream exiting thesecond outlet 48 extends in a plane generally perpendicular to axis 17.Each second outlet 48 is arranged to direct the exiting air to flowacross fuel support surface 30 or plate 125 substantially along dottedline 35 as shown on FIG. 4. However, the direction of each exiting airstream is slightly shifted so that the stream is not directed to passover axis 17. Air injector 34 identified 34-1 in FIG. 4 is substantiallydiametrically opposite to air injector 34-6. The direction of the airexiting second outlet 48 of injector 34-1 is directed to impinge oninner wall 36 midway between air injectors 34-5 and 34-6. Similarly, thedirection of flow from the second outlet 48 of air injector 34-2 isacross the fuel support surface to a point midway between injectors 34-6and 34-7. Thus, the air flow of each injector is directed to the left ofcentral axis 17, thereby creating a swirl within the combustion chamber.The flow pattern developed by the plurality of exiting air streams fromthe second outlets 48 thus develops a high pressure zone indicatedgenerally by the circle 60.

Arrangements of the parts of the combustion chamber may be more clearlyunderstood from reference to FIGS. 1 and 9. Housing 16 is generallycylindrical and has a central axis 17. The primary combustion zone islocated immediately above fuel support surface 30 (FIG. 9) or plate 125(FIGS. 1 and 5). Combustion takes place within the volume 70 defined bythe tubular housing 16 between fuel support surface 30 or plate 125 andheat sink 12. Housing 16 comprises an annular restriction ring 72coupled to inner surface 36 of housing 16. Annular ring 72 comprises acentral aperture 74 preferably concentric with axis 17.

Housing 16 has an upper edge 80. Thus, the axial length of thecombustion chamber contained within by the housing 16 is the lengthbetween fuel support surface 30 (FIG. 9) or plate 125 (FIGS. 1 and 5)and heat sink 12. The location of restriction ring 72 within housing 16is such that optimum flame height and heat transfer to the heat sink areachieved.

Above ring 72, housing 16 comprises a thermal break at 82 that may be inthe form of an air gap with the portion of the housing above the air gapbeing separated from the portion below the air gap by relatively narrowmetallic components. Alternatively, the thermal break may be in the formof a ceramic or other material that would inhibit the flow of heat fromthe upper portion of the combustion chamber to the lower portion thereofbelow the thermal break.

Housing 16 may further comprise a plurality of pressure releaseapertures 84, preferably holes, provided circumferentially through thewall of housing 16. These are located above thermal break 82 but belowupper edge 80 of housing 16.

As shown in the drawings, apparatus 10 comprises a heat sink support 18located on the housing 16 adjacent the upper edge 80. In a preferredembodiment, heat sink support 18 comprises a plurality of metallic railsprojecting slightly above upper edge 80 and coupled to the outer surface45 of housing 16. Furthermore, a flat annular ring 86 is coupled toupper part 80 of housing 16. The beforementioned rails are arrangedcircumferentially around housing 16 and serve to support heat sink 12.When heat sink 12, in this case a cooking utensil, is placed oversupport 18, a heat transfer gap 90 is defined. Combustion products whichtravel upwardly within housing 16 impinge directly upon heat sink 12 andthen pass through heat transfer gap 90 to exit from heating apparatus10. As the combustion gases pass through heat transfer gap 90, they areforced to travel along a portion of the periphery of heat sink 12 movingradially outwardly along the bottom surface of heat sink 12. Tofacilitate this heat transfer process, heat sink 12 is preferably largerthan the diameter of housing 16. Thus, heat transfer gap 90 iseffectively toroidal in shape.

As seen in FIG. 5, the dotted lines indicated at 100 represent the flowof air passing out through second outlets 48 of air injectors 34. As theair exits second outlets 48, it travels substantially parallel to theplane of plate 125. As air impacts on the fuel or other air streams froman opposing injector 34, it swirls and passes upwardly in the combustionchamber. This swirling or turbulent air will be mixed with the gasesreleased by the burning fuel and will form the combustion products.

Dotted lines 102 illustrate the air flow pattern for the air exitingfirst outlet 46 of air injectors 34. The air flow from aperture 130provides additional air needed for a better combustion at the centralzone of the combustion chamber. The air flowing in the pattern 102comprises the air exiting first outlets 46 of air injectors 34. The airflowing out of first outlets 46 thus forms a substantially cylindricalair envelope. That air envelope exits first outlets 46 travellingsubstantially parallel to axis 17 of housing 16. The air stream 102 thenimpinges upon restriction ring 72, which causes the air flow 102 todivert slightly radially inwardly to pass through a circular aperture 74defined by the restriction ring 72. Thereafter, the air flow bendsradially outwardly and passes axially upwardly along housing 16.

Air flow pattern 102 thus forms an envelope confining the combustiongases generated by air flow 100 and combustion products released fromthe fuel on fuel support 30. Air flow 102 is thought to serve threepurposes. Firstly, the air envelope provides an envelope for theswirling combustion gases above the fuel. Secondly, it provides acooling effect limiting heat transfer to housing 16. And thirdly, itassists in transferring heat to the heat sink. The air stream coming outof aperture 126 cools plate 125 and mixes with the air flow incomingfrom apertures 128 to produce air stream 132. The presence of air stream132 between housing 16 and external tubular member 120 further reducesheat transfer to the external surface thereof.

The air stream coming out of aperture 130, and air streams 100 and 102,together with the gases released during the combustion process of thefuel, travel upwardly and impinge upon heat sink 12. Thereafter, thegases exit housing 16 by passing through heat transfer gap 90 and inaddition, to a minimal extent, through the pressure release apertures84, if any are present. In order to favor the heat transfer to the sink,the total area available for flow through heat transfer gap 90 ispreferably larger than the area of aperture 74 defined by restrictionring 72. In addition, little gas passes through apertures 84, if any, torelieve the pressure at the upper portion of the combustion chamber,which may thus increase the temperature in the area just below the heatsink.

The present invention enables extremely efficient heating of the heatsink for a number of different reasons. Fan 20 forces air into thecombustion zone through air injectors 34. This means that the fuel isburnt in an area being at a pressure higher than ambient pressure. Thus,burning of the fuel occurs under pressures slightly higher than ambient.The higher pressure in the area of the fuel appears to provide a loweredignition and combustion temperature, which in turn means that the rateof gasification from the fuels is slowed. In addition, the cylindricalair curtain formed by flow pattern 102 cools the combustion gases ascombustion continues, keeping the combustion gases below the temperatureat which NO_(x) and other pollutants are generated. This also helps inproviding a more complete combustion of the fuel.

Restriction ring 72 within housing 16 assists in the transfer of thecombustion gases released in what may be considered to be a primarycombustion zone, and in moving the heat released directly towards theheat sink. Because of the location of the restriction ring 72, there isa tuning effect within the length of the combustion chamber that alsoappears to favor movement of the heat generated by combustion in thegeneral direction of the heat sink. The uppermost portion of housing 16will be at the lowest temperature. Any heat lost through the wall ofhousing 16 represents loss of heat that otherwise should be directedtoward heat sink 12. Thermal break 82 tends to minimize the transfer ofheat from the upper portion of housing 16 to the lower portion thereofand air flow pattern 102 tends to move heat from the combustion gasesquickly and efficiently upwardly toward the heat sink so that the amountof heat lost through the wall of housing 16 is reduced.

The present invention provides particularly efficient cooking usingsmall blocks of wood. To start the use of the apparatus, a small pieceof wood or kindling is placed on fuel support surface 30 or plate 125inside the circular array formed by injectors 34. After initiation offire, fan 20 is turned on, and housing 16 is placed over the supportsurface. If a tubular member 120 is present around housing 16, both ofthese are placed over the support surface jointly. As the wood piece isconsumed, more wood can be added, for example through an aperture 110located above restriction ring 72. The amount of air to be delivered inthe chamber is adjusted with an air flow valve (not shown).

As shown in FIG. 3, the area of first outlet 46 of each injector 34 ispreferably considerably smaller than the area of second outlet 48.However, outlet 48 is positioned substantially perpendicular to bore 42passing centrally through the body 40 of the injector. It is desiredthat the air passing through the first outlet 46 be moving relativelyquickly, and that sufficient air passes through the plurality of firstoutlets 46 to form the air flow pattern 102.

Housing 16 and air chamber 14 is preferably made of a highly thermalconductive material such as stainless steel. Fuel support surface 30 andplates 124 and 125 may also be made of stainless steel. Further, airinjectors 34 should be evenly distributed about a circular array and thedistance from the array to the interior surface of housing 16 isapproximately 1 mm to ensure an efficient swirl within the combustionchamber. The air injectors themselves may also be made from stainlesssteel. Experimental evidence shows that preferred air flow and air flowpatterns are achieved when the area of first outlets 46 compared to thearea of second outlets 48 is between 12 and 18%. It is considered thatthe area of second outlets 48 compared to first outlets 46 may be ashigh as 20 to 1. Of course the ratio of areas could be considerablyless.

An example of a fan 20 suitable for the purposes of the presentinvention is a 4715FSB30™ manufactured and sold by NMB Technologies.Preferably, fan 20 is isolated from the heat generated by the fuelburning on the support surface. In addition, to minimize heat flowconducted along the wall of air inlet chamber 14, the housing for fan 20may be spaced from the air inlet chamber 14 by an air gap, therebyadding to the thermal isolation of the fan.

One of the more interesting observations is that there does not appearto be any substantial flow through aperture 110 provided in housing 16for addition of fuel. As shown in FIG. 5, the flow pattern 102 bendsinwardly, upwardly of the restriction ring 72. It has been observed thatessentially no flame passes outwardly through the open aperture 110.Similarly, there is been no substantial flow through pressure reliefapertures 82. The flow of gases travelling through the restriction ring,Q_(ring), is thus equal to the area of the ring, A_(ring), multiplied bythe velocity at which the gases are travelling through the ring.

Heat sink support elements 18 support heat sink 12 so as to define a gapbetween the heat sink and the housing. Because of the configuration ofthe present device, the gases at the exit are travelling slightly slowerthan at the ring as they pass through aperture 74. The exit velocitythrough the thermal transfer gap 90 can be reduced is further byincreasing the area of the thermal transfer gap 90 while keeping thedistance between the support surface 30 and the lower surface of theheat sink 12 constant.

In understanding the processes occurring within the combustion chamber,this might more easily be explained and understood as a fluid dynamicsprocess.

The cooling air element illustrated by the flow pattern 102 desirablytravels upwardly at approximately the same speed as the combination offlame and combustion gases. The exit speed of the combined gases throughthe thermal transfer gap 90 is reduced slightly to allow the heat toremain as long as possible adjacent the base of the heat sink. The swirlgenerated inside the combustion chamber causes the flame and combustiongases to remain substantially within the central portion of housing 16,thereby concentrating the greatest portion of the heat centrally of theunder side surface of the heat sink. Upon impinging the heat sink, theheat is therefore substantially uniformly diffused on the surfacethereof.

In the device of the present invention, the maximum temperature measuredinside the combustion chamber when operating with wood as the fuel, was950° C. at the centre of support surface 30.

While the invention has been described in connection with specificembodiments thereof, it will be understood that it is capable of furthermodifications, and this application is intended to cover any variations,uses or adaptations of the invention following, in general, theprinciples of the invention, and including such departures from thepresent description as come within known or customary practice withinthe art to which the invention pertains, and as may be applied to theessential features hereinbefore set forth, and as follows in the scopeof the appended claims.

What is claimed is:
 1. A heating apparatus for generating andtransferring heat to a heat sink comprising: a tubular combustionchamber for combusting a fuel therein, thereby generating heat, thecombustion chamber comprising a lower portion provided with a bottomsurface, and an open upper portion; air injecting means coupled to thecombustion chamber for injecting air therein while the fuel iscombusting, the air injecting means being coupled to the combustionchamber in a manner such that upon injection of air, a first flow of airsubstantially swirls towards the upper portion of the combustion chamberand a second flow of air flows substantially parallel to an inner wallof the tubular combustion chamber towards the upper portion whereby theheat is transferred to the heat sink in intimate contact with thecombustion chamber; the combustion chamber is disposed over the airinjecting means, and said air injecting means is comprised of a seriesof air injectors extending from an air inlet chamber into the combustionchamber.
 2. The apparatus according to claim 1 wherein the series of airinjectors are provided at an equal distance from each other around thecombustion chamber.
 3. The apparatus according to claim 2 wherein eachair injector comprises an inlet wherein the air is injected from the airinlet chamber, a first outlet and a second outlet, the first outletallowing injection of air towards the upper portion of the combustionchamber in a manner substantially parallel to the inner wall thereof,and the second outlet allowing injection of air towards the middle ofthe combustion chamber in a manner substantially parallel to the bottomsurface of thereof.
 4. The apparatus according to claim 3 wherein thediameter of the first outlet is smaller than the diameter of the secondoutlet.
 5. The apparatus according to claim 1 further comprising asupport coupled to the upper portion of the combustion chamber, and thesupport being adapted to receive the heat sink thereon.
 6. The apparatusaccording to claim 5 wherein the support extends over an upper rim ofthe upper portion of the combustion chamber.
 7. The apparatus accordingto claim 6 wherein the combustion chamber comprises a series ofapertures in the inner wall thereof, and the apertures beingsubstantially equally spaced apart circumferentially and being providedbetween the upper edge of the combustion chamber and an upper rim of thesupport.
 8. A process of generating and transferring heat comprising thesteps of: providing a combustion chamber, the combustion chambercomprising an open upper portion and a bottom surface, and wherein afuel is provided therein; injecting air into the combustion chamberwhile the fuel is combusting, the air being injected in a manner suchthat a first flow of air substantially swirls towards the upper portionof the combustion chamber and a second flow of air flows substantiallyparallel to an inner wall of the combustion chamber towards the upperportion thereof whereby the heat is transferred to a heat sink inintimate contact with the combustion chamber; said air injection iscarried out from underneath the combustion chamber through a series ofair injectors extending from an inlet air chamber into the combustionchamber, the air injectors being disposed at an equal distance from eachother around the bottom surface of the combustion chamber; each of saidinjectors comprises an inlet wherein the air is injected from the airinlet chamber and a first outlet and a second outlet, the first outletallowing the injection of air towards the upper portion of thecombustion chamber is a manner substantially parallel to the inner wallthereof, and the second outlet allowing the injection of air towards themiddle of the combustion chamber in a manner substantially parallel tothe bottom surface of the combustion chamber.
 9. The process accordingto claim 8 wherein the diameter of the first outlet is smaller than thediameter of the second outlet.
 10. The process according to claim 8wherein the inner wall comprises a ring extending perpendicularlythereof, thereby forming an opening having a diameter smaller than thediameter defined by the upper portion of the combustion chamber.
 11. Theprocess according to claim 8 wherein the heat sink is provided on asupport coupled to the upper portion of the combustion chamber, and thesupport being adapted to receive the heat sink thereon.
 12. The processaccording to claim 8 wherein the combustion chamber comprises a seriesof apertures in the inner wall thereof, and the apertures beingsubstantially equally spaced apart circumferentially and being providedbetween an upper edge of the combustion chamber and an upper rim of thesupport.